Device and method for evaluating a useful signal originating from a proximity sensor

ABSTRACT

The invention relates to a device and method for evaluating a useful signal U(t) originating from a proximity sensor. The device comprises a first recognition means ( 17 ) which changes the switching state thereof when the value (U 1 (t)) of a first signal produced from the useful signal exceeds a first limit value that characterizes the proximity of an object. In addition, a second recognition means ( 34 ) is provided which changes the switching state thereof from a first to a second state when the value (U(t)) of the useful signal exceeds a second limit value that characterizes the removal of an object. A decision means ( 32 ), which is connected to the first recognition means ( 17 ) and to the second recognition means ( 34 ), changes its switching state when the first recognition means ( 17 ) changes the switching state thereof from its first to a second switching state, and the second recognition means does not change the switching state thereof during a predetermined first time span (Δt 1 ) after the switching state of the first recognition means is changed.

REFERENCE TO RELATED APPLICATIONS

The present application claims priority of German application 100 01943.9, filed on Jan. 18, 2000, the disclosure content of which is herebyexpressly also made the object of the present application.

FIELD OF THE INVENTION

The invention relates to a device as well as a method for evaluating auseful signal originating from a proximity sensor, more especially froman opto-electronic proximity sensor.

BACKGROUND TO THE INVENTION

In almost all electric or electronic devices, manual operation iseffected by switches. These switches are almost always designedmechanically, two metal parts being brought into contact or respectivelyout of contact in order to close or respectively to open a circuit.However, this mechanical design has the disadvantage, amongst others,that it has mechanical wearing parts and consequently only has a limitedservice life and is fundamentally water-sensitive, such that, whererequired, a costly casing is necessary.

Optical switches are already known; however, up to now they have beenextremely lavish and, consequently, expensive and do not yet have therequired standard of operational reliability. However, in principle,optical switches have advantages, as they manage, generally speaking,without any moving mechanical parts and the switching process can betriggered by mere tapping or by contacting a control surface or bysimply through approximation to a sensor.

Furthermore, so-called proximity sensors or also rain sensors are knownin the technology, by means of which the displacement of an object ontoa surface or the contacting or wetting of a surface can be detected,devices being known in these cases which output a signal containing dataregarding the direction of displacement and the speed of displacement ofthe object. Such a device is known, for example, in WO 95/01561.

OBJECT OF THE INVENTION

It is the object of the invention to make available a device orrespectively a method for evaluating a useful signal originating from aproximity sensor, by means of which signal an optical switch can beoperated. Optical switch in this case means that through theintermediary of a defined displacement, for example of a finger, adefined switching process is triggered, that is to say more especiallythe interrupting or respectively closing of an electric circuit.

SUMMARY THE INVENTION

These and other objects are achieved according to the invention by theprovision of a device for evaluating a useful signal having a value, theuseful signal originating from a proximity sensor, wherein the value ofthe useful signal changes with an approach of an object to the proximitysensor and with a removal of the object from the proximity sensor andwherein direction and amount of this change are characteristic of atleast one of direction, speed, and distance of the object. The deviceincludes a first recognizing means having an output which changes from afirst switching state to a second switching state when a first value ofthe useful signal, or a first value of a first signal generated from theuseful signal, exceeds or falls below a first limit value which ischaracteristic of the object approaching the proximity sensor; a secondrecognizing means having an output which changes from a first switchingstate to a second switching state when a second value exceeds or fallsbelow a second limit value, which is characteristic of at least one ofthe object approaching the proximity sensor and the object being removedfrom the proximity sensor; a first deciding means coupled to the outputof the first recognizing means and the output of the second recognizingmeans, and having an output which changes from a first switching stateto a second switching state when the first recognizing means changes itsswitching state and when the second recognizing means maintains itsswitching state, and wherein the first deciding means remains in thesecond switching when the second recognizing means maintains itsswitching state for a predetermined first time slot after the switchingstate of the first recognizing means has been changed; a time detectioncircuit having an output, the time detection circuit being coupled tothe output of the first deciding means, the output of the time detectioncircuit being set to an active state when the output of the firstdeciding means is set to an active state for a time slot which is longerthan the predetermined time slot; and a second deciding means coupled tothe output of the time detection circuit and being set when the outputthe time detection circuit is set to its active state. Modificationswill be readily apparent to those skilled in the art, and thisapplication is intended to cover any adaptations or variations thereofThe exemplary embodiment and other examples given in this applicationare examples only and not intended to limit the scope of the invention.

To better represent the invention, the arrangement described in WO95/01561 is initially represented in brief, the disclosure content ofwhich is hereby expressly also made an object of the presentapplication. However, it must be stressed that the present invention isnot restricted to operating only with the arrangement described in thiscase, but can operate whenever there is a useful signal, which containsspeed data and direction data on a moving object. This speed data, as arule, is only available in one dimension and this is also sufficient forthe operation of the device according to the invention, howevermulti-dimensional data can also be evaluated. In the present case, theuseful signal is an analogue voltage signal, but this is not urgentlynecessary. It could also be, for example, a digitalized signal.

Example of an Applicable Proximity Sensor

FIG. 7 shows a proximity sensor, as is known substantially in WO95/0156 1. At least two light-emitting diodes 1, 3 are disposed under aglass plate 31, the light of which light-emitting diodes can bereflected at least partially at the glass plate 31 and fall on thephotodiode 2. A correspondingly set-up light-emitting diode can also actas photodiode. The glass plate or another surface should be translucentto light at least in a certain wavelength range. The light emitted fromthe light-emitting diode 3 does not act as a measuring section in thiscase, but is only required to compensate for the external light. It isconsequently conceivable and in many cases expedient to block the lightpath of this light-emitting diode to the effect that it cannot penetrateto the outside. An arrangement 116 for blocking one of the two beams oflight is represented in FIG. 9. It is also conceivable to configure thefirst light-emitting diode 1 as a long range emitting laser diode withradiation region b, and to configure the second light-emitting diode asa light-emitting diode, which only emits in the short range, withradiation region a (FIG. 8). In addition, the light-emitting diodes canbe separated from one another by a separating wall 113 in the housing104. This is a modification of the arrangement described in WO 95/01561, which can be sensible for the present purpose.

The light of the light-emitting diode 1 is only partially reflected atthe glass pate 31 and consequently passes to the outside, the lightbeing reflected, in its turn, by an object, in this case a finger, andconsequently being able to be diffused back at least partially into thephotodiode 2. The two light-emitting diodes are supplied with voltage bymeans of a clock generator 13, the voltage signal of one of the twolight-emitting diodes being inverted. Where the light output of thediodes is even and where the reflection is precisely symmetrical orwhere there is suitable regulation of the light brightness of at leastone of the two diodes (see below), there is a direct current voltagesignal at the output of the photodiode 2, which direct current voltagesignal is supplied to a high-pass filter 32 to eliminate direct currentvoltage and low-frequency alternating-current portions. The high-passfilter 32, the cut-off frequency of which is below the frequency of theclock generator 13, sets the signal supplied to it to “0” as long as itis a direct current voltage signal. Influences from external lightsources are excluded with this arrangement.

This signal, filtered in this way, is supplied to an amplifier 4 andthen to a synchronous demodulator 5. The synchronous demodulator 5receives its clock signal from the frequency generator 13, this clocksignal being delayed accordingly by the delay unit 15 for adaptation tothe signal run times in the high-pass filter 32 and in the amplifier 4.The synchronous demodulator 5 divides the signal of the light sources 1and 3, which is common to the signal path of the light receiver 2, ofthe high-pass filter 32 and of the amplifier 4, back into two separatepaths. The signal sections cut out by the synchronous demodulator 5, arecleaned in the low pass filters 6 and 7 of nuisance spectral regions andare supplied to the comparator 9. In the case represented, thecomparator 9 comprises a simple operation amplifier. The differencevalues corresponding to the light emitters are at the outputs of therespective low pass filters 6 and 7. In the correspondingly tuned state,that is two times the value zero. These two signals are supplied to thecomparator 9. The voltage value U(t), the useful signal, is at theoutput of this comparator. This signal is also supplied to the signalcentering level 11 via a low pass filter 10.

The output of the signal centering level 11 is connected to a regulator12, which regulates the signal voltage for the light-emitting diode 3.The achievement of this arrangement is that the useful signal changeswhere there is a change in the reflection of the light beam emitted bythe light-emitting diode 1, however is always continuously returned tozero value. The time constant for this resetting is determined in theexemplified embodiment by the low-pass filter 10.

It is an object of the invention to use the useful signal U (t) to theeffect that a defined switching process is triggered by a defineddisplacement of an object. In the present example, the tapping of afinger, of a hand or of another part of the body of the user onto asensor-active region S on the glass plate 31 is to be detected and aswitching process consequently triggered. However, other applicationsare conceivable, where the displacement of a mechanical element, forexample a so-called “jumping jack”, is to be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now explained in more detail by way of an exemplifiedembodiment. In which:

FIG. 1 a is a switching arrangement for the evaluating according to theinvention of a useful signal,

FIG. 1 b is a complete arrangement of an optical switch,

FIG. 2 is the curve of the useful signal U(t) when the sensor-activeregion is tapped,

FIG. 3.1 is the curve of the measuring signal when the sensor-activeregion is wiped over,

FIG. 3.2 is the curve of the useful signal, if, for example, a cloth ismoved rapidly to and fro on the glass plate,

FIG. 4 a is the curve of the useful signal U(t) when the sensor-activeregion is tapped,

FIG. 4 b is the curve of the differentiated displacement signal U₁(t)when the sensor-active region is tapped,

FIG. 5 a is the curve of the measuring signal when the sensor-activeregion is wiped over,

FIG. 5 b is the curve of the output signal of the first threshold valueswitch in the situation represented in FIG. 5 a,

FIG. 6 is the signal curves of U₂₀(t) and U_(R)(t), as well as thestored value U_(R)(t₀),

FIG. 7 is a proximity sensor according to the state of the art,

FIGS. 8, 9 are possible modifications to the proximity sensor in FIG. 7

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is now described in more detail as an example withreference to the enclosed drawings. However, the exemplified embodimentsare only examples, which are not to restrict the inventive concept toone certain arrangement.

The useful signal U(t) output by the sensor device described above isrepresented in various situations in FIGS. 2, 3.1, and 3.2. FIG. 2records the useful signal U(t) when the sensor-active region S istapped. A switching process is to be triggered by this type of signal.Useful signal curves are recorded in FIG. 3.1 and FIG. 3.2, as occurwhen the sensor-active region S is wiped over once or respectively whenit is wiped over to and fro. These types of signal curves are not totrigger any switching processes. This goal is achieved with thisembodiment as follows (FIG. 1 a):

The useful signal U(t) is supplied to the high-pass filter 16, whichworks here as differentiator, such that the value U₁(t) of thedifferentiated displacement signal is situated at the output of thehigh-pass filter. Where an object, for example a finger, is displacedonto the sensor-active surface of the glass plate 31, the value U(t) ofthe useful signal increases slowly analogous to the displacement andstops suddenly when the finger is braked on the glass plate 31, seeFIGS. 2 and 4 a. If the finger remains and does not move, the value U(t)of the useful signal is regulated slowly back to U₀. The sudden changein value of the useful signal results at the output of the high-passfilter 16 in a jump in the value of the displacement signal U₁(t), seeFIG. 4 b. This is detected by the threshold value switch 17 when apredetermined negative value U_(G1) is exceeded and the output of thefirst threshold switch 17, which is connected to the set-input of thefirst flip-flop 32, is set to active and consequently the firstflip-flop 32 is set. The cut-off frequency of the high-pass filter 16 isselected such that a tapping at moderate speed still results in aneasily detectable signal. The cut-off frequency could, for example, bein the range of 100 Hertz.

A signal generated from the useful signal is used therefore in thiscase, that is to say the displacement signal obtained throughdifferentiation, and this triggers a first process when its value U₁(t)exceeds a certain limit value U_(G1). Switching arrangements and casesof application are also conceivable, however, where the useful signal isused directly and triggers a process—change in flip-flop state in thiscase—, when the value U(t) of the useful signal exceeds a certain valueor falls below a certain value.

Every displacement, which is quick enough and covers the firstsensor-active region, triggers this process, i.e. the output of thefirst flip-flop 32 is initially set to active. A wiping movement orsimilar is also sufficient to do this, but it is not, however, to berecognized as a deliberate switching process (see FIGS. 3.1 and 3.2).This is why the useful signal is supplied to a second threshold switch34, which becomes active when the value U(t) of the useful signal fallsbelow a certain second threshold value U_(G2). The fact that theremoving of an object (removal of a finger) results in a reducing ofU(t) in the opposite direction in comparison with the approximation ismade use of here, in the example in the negative range (FIG. 3.1). Wherethe second threshold value UG₂ of the second threshold value switch 34is exceeded in the negative direction, its output U₃₄(t) is set toactive (see FIG. 5).

Since the output of the threshold value switch 34 is connected to thereset-input of the flip-flop 32, setting this output to active willreset the flip-flop 32. Therefore, when there is a wiping or similarmovement which has set the flip-flop to active, flip-flop 32 is reset tozero a short time later. This means that the output of the flip-flop 32is reset back to zero. The output signal of the flip-flop 32 is thensupplied to the time detection circuit 33. The time detection circuit 33is set up such that its output is only set to active if the flip-flop 32has been active longer than a predetermined time Δt₁, for example 100ms. This predetermined first time slot Δt₁ corresponds substantially tothe normal minimum dwell time of a finger, a hand or another part of thebody when tapping a switch, which is configured as an electric switchingelement.

The output of the time detection circuit 33 is connected to theset-input of the second flip-flop 18. Where there is a deliberatetapping of the sensor-active surface, the output of the second flip-flop18 is consequently set to active, as in this case the time betweensetting the first flip-flop 32 and resetting this flip-flop is greaterthan Δt₁, in other words: The finger remains longer than Δt₁ on thesensor-active region S. However, where there are movements which are notto trigger any switching process—for example wiping over with a cloth—,the time between setting and resetting the first flip-flop 32 is smallerthan Δt₁, such that these movements do not result consequently in thesecond flip-flop 18 being set. Therefore, by tapping the sensor-activesurface, therefore, the state of the second flip-flop 18 is changed in acontrolled manner. The output of the flip-flop 18 can also be connectedto a switch 23, for example a relay.

In many application cases it is desirable for the second flip-flop 18,which is set through the tapping of the sensor-active region S, to bereset again by targeted removal of the finger. This then produces thefunction of a key. However, it is advantageous when the clearing of theflip-flop 18 is not achieved until the finger has been removed a fewmillimeters from the glass plate 31, so as to prevent the flip-flop frombeing cleared inadvertently through a minimal displacement. This problemis solved as follows in the exemplified embodiment represented in thiscase:

The instantaneous value of the control signal U_(R)(t), which issituated at the output of the operation amplifier 11, is scanned andstored at a moment at which the approximating object is still situatedjust in front of the operator interface. To achieve this, this signal issupplied to the delay circuit 20. The voltage value U₂₀, which issituated at the output of the delay circuit 20, is stored in the memory21 at the moment t₀ at which there is a signal situated at the output ofthe first threshold value switch 17, that is to say at the moment atwhich the first threshold value switch 17 has recognized the moment ofthe tapping. The value U_(R)(t₀), stored in this way, is supplied to afirst input of the comparator 22. The control signal with the valueU_(R)(t) is located at the second input of the comparator. As long asthe value of the control signal is above the value at the output of thememory 21, the comparator circuit 22 does not supply an output signal.However, if the value of the control signal at moment t₁ falls below thestored value, the output of the comparator is set to active. The signalsU₂₀, U_(R)(t) and U_(R)(t₀) are represented in FIG. 6. The secondflip-flop 18 is reset with this signal.

The principle of the invention is fundamentally based on evaluating auseful signal originating from a proximity sensor, more especially froman opto-electronic proximity sensor, the value U(t) of which usefulsignal changes when an object is moved nearer to the proximity sensorand when it is removed away from the proximity sensor and the directionand amount of this change are characteristic of the direction and speedand/or distance of the object.

To this end, a first recognizing means 17 changes its switching statefrom a first state to a second state when the value U(t) of the usefulsignal or a value U₁(t) of a first signal generated from the usefulsignal exceeds or falls below a first limit value which ischaracteristic of the approximation of an object. A second recognizingmeans 34 changes its switching state from a first state to a secondstate when the value U(t) of the useful signal or the value of a secondsignal generated from this useful signal exceeds or falls below a secondlimit value which is characteristic of the removal of an object.Connected to the recognizing means 17, 34 is a deciding means 32, whichchanges its output state from a first state to a second state when thefirst recognizing means 17 changes its switching state from its firststate to its second switching state and the second recognizing meansdoes not change its switching state within a predetermined first timeslot Δt1 once the switching state of the first recognizing means hasbeen changed.

The principle can include more elements in a further development. Forexample, in this way a third recognizing means resets the output stateof the deciding means from the second state back to the first outputstate when the value U(t) of the useful signal or the value U₃(t) of athird signal generated from this useful signal falls below or exceeds athird limit value which is characteristic of the removal of the object.The third limit value is preferably generated from the time curve of theuseful signal or of a signal generated by the useful signal. This thirdlimit value corresponds to the value U(t) of the useful signal or of asignal generated by the useful signal at a moment, which lies a certainpredetermined time slot before the moment (t₀) of the changing of thestate of the first recognizing means (17). This can be achieved by afixed part factor or by a time-delaying of the control signal U_(R)(t),so as to be independent of appearances of wear and tear, for example, onthe surface of the switch. Useful signal, first signal and second signalare preferably analogue voltage signals.

In the deciding means the first recognizing means 17 sets a firstflip-flop 32 and the second recognizing means 34 resets the firstflip-flop 32. A time detection circuit 33, which is connected to theoutput of the first flip-flop 17, is set to active when the out-put ofthe first flip-flop 32 has been set to active for a time slot which islonger than the predetermined time slot Δt₁. The output of the timedetection circuit 33 sets a second flip-flop 18.

This can consequently be used to form an opto-electronic switch, whichis equipped with at least one light-emitting transmitting element and atleast one receiving element. The receiving element outputs its signals,the value of which depends on the amount of light received, to anevaluation unit, in which at least one switching element changes itsswitching state when the value of the first signal, or the value ofanother signal derived from this signal, exceeds or falls belowpredetermined limit values. Transmitting and receiving elements can bedisposed in such a manner that the light coming from the transmittingelement is diffused or reflected by objects, which are located within acertain region, or by a displaceable element, which is at apredetermined spacing from the receiving element and the transmittingelement, such that at least one portion of this diffused or reflectedlight reaches the receiving element. Consequently, the change in theamount of reflected or diffused light, which is received by thereceiving element, caused by a displacement of the object or by adisplacement of the displaceable element, causes a change in state ofthe switching element if the displacement is inside the limits of apredetermined displacement pattern.

This displacement pattern is preferably a tapping of a defined region byfinger, hand or another part of the body. For example, a defined regionon a glass or plexiglass pane or on a photoconductor, which is coupledto the transmitting element and/or receiving element, can be tapped.

The addressed displaceable element can, for example, be a snap-typespring, as is sometimes used in conventional switches. The recognizingmeans either recognizes just the displacement pattern of this snap-typespring on its own or in addition to the approximation of the object. Forexample, the snap-type spring can be situated on the proximity sensor soas to show the user the switching effect in a tactile manner, howeverjust the displacement of the displaceable element on its own can also bedetected and evaluated. The snap-type spring is displaceable against arestoring force and can, for example, overcome a dead point when movingagainst the restoring force.

It will be appreciated by one skilled in the art that this descriptioncan be subject to the most varied modifications, changes andadaptations, which range in the region of equivalents to the attachedclaims.

This displacement pattern is preferably a tapping of a defined region byfinger, hand or another part of the body. For example, a defined regionon a glass or plexiglass pane or on a photoconductor, which is connectedto the transmitting element and/or receiving element, can be tapped.

The addressed displaceable element can, for example, be a snap-typespring, as is sometimes used in conventional switches. The recognizingmeans either recognizes just the displacement pattern of this snap-typespring on its own or in addition to the approximation of the object. Forexample, the snap-type spring can be situated on the proximity sensor soas to show the user the switching effect in a tactile manner, howeverjust the displacement of the displaceable element on its own can also bedetected and evaluated. The snap-type spring is displaceable against arestoring force and can, for example, overcome a dead point when movingagainst the restoring force.

1. Device for evaluating a useful signal having a value, the usefulsignal originating from a proximity sensor, wherein the value of theuseful signal changes with an approach of an object to the proximitysensor and with a removal of the object from the proximity sensor andwherein direction and amount of this change are characteristic of atleast one of direction, speed, and distance of the object, comprising: afirst recognizing means having an output which changes from a firstswitching state to a second switching state when a first value of theuseful signal, or a first value of a first signal generated from theuseful signal, exceeds or falls below a first limit value which ischaracteristic of the object approaching the proximity sensor; a secondrecognizing means having an output which changes from a first switchingstate to a second switching state when a second value exceeds or fallsbelow a second limit value, which is characteristic of at least one ofthe object approaching the proximity sensor and the object being removedfrom the proximity sensor; a first deciding means coupled to the outputof the first recognizing means and the output of the second recognizingmeans, and having an output which changes from a first switching stateto a second switching state when the first recognizing means changes itsswitching state and when the second recognizing means maintains itsswitching state, and wherein the first deciding means remains in thesecond switching when the second recognizing means maintains itsswitching state for a predetermined first time slot after the switchingstate of the first recognizing means has been changed; a time detectioncircuit having an output, the time detection circuit being coupled tothe output of the first deciding means, the output of the time detectioncircuit being set to an active state when the output of the firstdeciding means is set to an active state for a time slot which is longerthan the predetermined time slot; and a second deciding means coupled tothe output of the time detection circuit and being set when the outputof the time detection circuit is set to its active state.
 2. Deviceaccording to claim 1, wherein the proximity sensor comprises anopto-electronic proximity sensor.
 3. Device according to claim 1,further comprising a third recognizing means which resets the state ofthe second deciding means from the second switching state back to thefirst switching state when the value of the useful signal or the valueof a third signal generated from this useful signal falls below orexceeds a third limit value, which is characteristic of the removal ofthe object.
 4. Device according to claim 3, wherein the third limitvalue is generated from a time curve of at least one of the usefulsignal and a signal generated from the useful signal.
 5. Deviceaccording to claim 4, wherein the third limit value corresponds to thevalue of the useful signal or of a signal generated from the usefulsignal, at a moment which lies a predetermined time slot before thechanging of the switching state of the first recognizing means. 6.Device according to claim 1, wherein the second deciding means iscoupled downstream of the first and second recognizing means and furthercomprising an electric switching element coupled downstream of thesecond deciding means such that a change in the switching state of thesecond deciding means results in switching the electric switchingelement.
 7. Device according to claim 1, wherein the useful signal andthe first signal comprise analog voltage signals.
 8. Device according toclaim 1, wherein the first recognizing means comprises a high-passfilter, at the input of which is the useful signal, and a firstthreshold value switch, which is coupled downstream of the high-passfilter.
 9. Device according to claim 1, wherein the second recognizingmeans comprises a second threshold value switch, at the input of whichis the useful signal.
 10. Device according to claim 1, wherein the firstdeciding means comprises a first flip-flop having a set input which iscoupled to an output of a first threshold value switch and a reset inputwhich is coupled to an output of a second threshold value switch; thetime detection circuit comprises an input coupled to an output of thefirst deciding means, and an output set to active when the output of thefirst deciding means has been set to active for a time slot which islonger than the predetermined time slot; and the second deciding meanscomprises a set input coupled to the output of the time detectioncircuit.
 11. Device according to claim 1, wherein the object comprisesat least one of a finger, a hand, and a body part of a user, and whereinthe predetermined first time slot corresponds to a normal dwell time ofat least one of the finger, the hand, and the body part when tapping aswitch which is configured as an electric switching element.
 12. Deviceaccording to claim 3, wherein the third recognizing means comprises: adelay circuit having an input for receiving a third signal generatedfrom the useful signal, the delay circuit having an output; a memoryhaving a first input coupled to the output of the delay circuit, asecond input coupled to the first recognizing means, and an output; anda comparator having a first input coupled to the third signal generatedfrom the useful signal, a second input coupled to the output of thememory, and an output coupled to a reset input of the second decidingmeans.
 13. Method for evaluating a useful signal originating from aproximity sensor, wherein a value of the useful signal or a value of another signal generated from the useful signal changes when an objectapproaches the proximity sensor and when the object is removed from theproximity sensor, and wherein direction and amount by which at least oneof the useful signal and the other signal changes are characteristic ofdirection, speed, and distance of the object, comprising the steps of:recognizing an approach of the object and switching a first recognizingmeans from a first switching state to a second switching state when thevalue of the first signal exceeds or falls below a first limit valuewhich is characteristic of the object approaching the proximity sensor;recognizing an approach of the object and switching a second recognizingmeans from a first switching state to a second switching state when thevalue of the other signal exceeds or falls below a second limit valuecharacteristic of the object approaching the proximity sensor; changingan output state of a first deciding meanswhich is coupled to at leastone of the first recognizing means and the second recognizing means whenthe first recognizing means changes switching states and when the secondrecognizing means maintains its switching state; recognizing a removalof the object by switching the second recognizing means; maintaining thesecond state of the deciding means when the second recognizing meansmaintains its switching state for a predetermined first time slot afterthe switching state of the first recognizing means has been changed;setting to active an output of a time detection circuit, which iscoupled to an output of the first deciding means, when the output of thefirst deciding means has been set to active for a time slot which islonger than the predetermined time slot; and coupling a second decidingmeans to the output of the time detection circuit and setting to activean output of the second deciding means, when the output the timedetection circuit is set active.
 14. Method according to claim 13,wherein the proximity sensor comprises an opto-electronic proximitysensor.
 15. Method according to claim 1, further comprising the step ofresetting, by way of a third recognizing means, an output state of thesecond deciding means from a second switching state back to a firstswitching state when at least one of (a) the value of the useful signaland (b) the value of a third signal generated from this useful signalfalls below or exceeds a third limit value which is characteristic of aremoval of the object.
 16. Method according to claim 15, furthercomprising the step of generating the third limit value from a timecurve of at least one of (a) the useful signal and (b) a signalgenerated from the useful signal, which lies a predetermined time slotbefore the changing of the switching state of the first recognizingmeans.
 17. Method according to claim 13, further comprising the step ofswitching an electric switching element when the switching state of thesecond deciding means changes.
 18. Method according to claim 13, furthercomprising the steps of: setting the first deciding means when an outputof the first recognizing means changes its switching state; resettingthe first deciding means when an output of the second recognizing meanschanges its switching state; setting to active an output of the timedetection circuit, which is coupled to an output of the first decidingmeans, has been set to active when the output of the first decidingmeans has been set to active for a time slot which is longer than thepredetermined time slot); and setting a second decision means when theoutput of the time detection circuit is set to active.